The present invention relates an audio amplifier circuit. In particular, it relates to the elimination, or at least reduction, of audible noise when power is supplied to or removed from an audio amplifier circuit.
Many audio amplifier circuits generate an audible sound when power is supplied to or removed from the audio amplifier circuit. Typically this noise is audible as a popping sound. It would be desirable to eliminate or at least reduce this noise.
A known audio amplifier circuit 1 for eliminating audible noise during system power on or off is shown in FIG. 1. The audio amplifier circuit 1 is implemented on an integrated circuit chip 2 and amplifies signals supplied to the input terminal 3. The output terminal 4 is connected to the load 5 through a capacitor 6 which is provided for DC isolation of the audio amplifier circuit 1.
For the elimination of noise, a node 7 between the external capacitor 6 and the load 5 is connected to earth through a switch 8 which is open during normal operation. Prior to the power being supplied to, or removed from, the audio amplifier circuit 1, the switch 8 is closed. Thus any noise signal generated subsequently in the audio amplifier circuit due to power supply switching is shorted through the switch 8 to earth and is consequently not passed as audible noise to the load 5.
However, the need to provide an external switch 8 externally of the integrated circuit 2 is disadvantageous. Firstly, the external switch 8 itself increases the circuit scale as an additional component is required. Secondly, the external switch 8 complicates the design of the overall circuit because of the need to control switching of the external switch 8 and to synchronize with the switching of the power supply to the integrated circuit 2.
It would be desirable to eliminate, or at least reduce, audible noise during power supply switching of an audio amplifier circuit without requiring the provision of an external switch.
According to a first aspect of the present invention, there is provided an audio amplifier circuit comprising: a voltage-gain amplifier and a current-gain amplifier in series thereafter between an input node and an output node; power supply lines for supplying power to the audio amplifier circuit; an outer negative feedback loop around the current-gain amplifier and the voltage-gain amplifier; an inner negative feedback loop around the voltage-gain amplifier, the current-gain amplifier being outside the inner feedback loop; first switch means for selectively connecting the power supply lines to the current-gain amplifier; second switch means for selectively switching the inner feedback loop or the outer feedback loop to the voltage-gain amplifier; and a control circuit arranged to control the switch means during power supply switching, wherein the control circuit is arranged to control the switch means when power is supplied by: initially opening the first switch means to disconnect the power supply lines from the current-gain amplifier; switching the second switch means to switch the inner feedback loop to the voltage-gain amplifier and subsequently, after a delay sufficient to allow the output of the voltage-gain amplifier to stabilise, closing the first switch means to connect the power supply lines to the current-gain amplifier; and switching the second switch means to switch the outer feedback loop to the voltage-gain amplifier.
As power is not supplied to the current-gain amplifier for an initial period sufficient for the output of the voltage-gain amplifier to stabilise, the settling transient of the voltage-gain amplifier is not output to the output node of the audio amplifier circuit. Of course it is subsequently necessary to switch the current-gain amplifier on. This is performed with the current-gain amplifier in open loop because the inner-feedback loop which does not include the current-gain amplifier is at that time switched to the voltage-gain amplifier. As a result, the supply of power to the current-gain amplifier does not create audible noise because its high bandwidth causes any transients to be above the audible range. In contrast, if the current-gain amplifier was in the feedback loop with the voltage-gain amplifier, the low bandwidth of the voltage-gain amplifier would slow the power-up creating audible noise. Subsequently, the outer-feedback loop is switched to the voltage-gain amplifier for normal operation with a feedback loop around both amplifiers.
Preferably, the audio amplifier circuit further comprises: a quiescent voltage supply for supplying a voltage at the quiescent level; and third switch means for selectively connecting the input node of the audio amplifier circuit or the quiescent voltage supply to the input of the voltage-gain amplifier, wherein the control circuit is arranged to control the third switch means when power is supplied by: prior to closing the first switch means, switching the third switch means to switch the quiescent voltage supply to the input of the voltage-gain amplifier; and on, or subsequent to, said switching of the second switch means to switch the inner feedback loop to the voltage-gain amplifier, switching the third switch means to switch the input node to the input of the voltage-gain amplifier.
This allows the voltage-gain amplifier to stabilise with the quiescent voltage being supplied to its input, thereby ensuring that it stabilises to its quiescent condition. The third switch means is subsequently switched to connect the input node to the voltage-gain amplifier for normal operation when the current-gain amplifier is switched on and the outer feedback loop around both amplifiers is switched to the voltage-gain amplifier.
Preferably, the audio amplifier circuit further comprises a charging path including resistance means for charging the output node of the audio amplifier circuit to its quiescent level through the resistance means; and a fourth switch means for selectively opening or closing the charging path, wherein the control circuit is arranged to control the fourth switch means when power is supplied by: closing the fourth switch means and subsequently, after a delay sufficient to allow the output node of the audio amplifier circuit to be charged to its quiescent level, opening the fourth switch means.
The charging path allows the output node of the audio amplifier circuit to be pre-charged to its quiescent level prior to the power supply being supplied to the current-gain amplifier. This means that on connection of the power supply to the current-gain amplifier its input and output are both at the quiescent voltage, thereby reducing the generation of audible noise on switching.
Desirably, the charging path is connected to a quiescent voltage supply for supplying a voltage at the quiescent level and the control means is arranged to control the fourth switch means when power is supplied by: initially opening the fourth switch means to isolate the output node of the audio amplifier circuit from the quiescent voltage supply; and performing said closing of the fourth switch means subsequently, after a delay sufficient to allow the output of the quiescent voltage supply to rise to its operating level.
This prevents the rise in the output of the quiescent voltage supply from creating audible noise at the output node of the audio amplifier circuit.
Preferably, the audio amplifier circuit further comprises a fifth switch means in series between the voltage-gain amplifier and the current-gain amplifier, and wherein the control means is arranged to control the fifth switch means when power is supplied by initially opening the fifth switch means to isolate the current-gain amplifier from the voltage-gain amplifier and subsequently, after a delay sufficient to allow the output of the power supply to rise to its operating level, closing the fifth switch means.
This prevents feedthrough of a signal from the voltage-gain amplifier during the rise in the level of the output of the power supply. Such a signal might otherwise drive the current-gain amplifier even whilst the power supply is disconnected therefrom and hence create audible noise at the output node.
Preferably, the audio amplifier circuit further comprises: a quiescent voltage supply for supplying voltage at the quiescent level to the forward path between the voltage-gain amplifier and the current-gain amplifier; a sixth switch means in series between the quiescent voltage supply and said forward path, wherein the control circuit is arranged to control the sixth switch means when power is supplied by: closing the sixth switch means to connect the quiescent voltage supply to the forward path and subsequently, after a delay sufficient to allow the output of the voltage-gain amplifier to stabilise, opening the sixth switch means.
This assists in causing the audio amplifier circuit to stabilise to its quiescent operating condition without the generation of audible noise, as a result of the output of the voltage-gain amplifier being driven to the quiescent voltage directly from the quiescent voltage supply.
Further, according to the first aspect of the present invention, there is provided an audio amplifier circuit comprising: a voltage-gain amplifier and a current-gain amplifier in series thereafter between an input node and an output node; power supply lines for supplying power to the audio amplifier circuit; an outer negative feedback loop around the current-gain amplifier and the voltage-gain amplifier; an inner negative feedback loop around the voltage-gain amplifier, the current-gain amplifier being outside the inner feedback loop; first switch means for selectively connecting the power supply lines to the current-gain amplifier; second switch means for selectively switching the inner feedback loop or the outer feedback loop to the voltage-gain amplifier; and a control circuit arranged to control the switch means during power supply switching, wherein the control circuit is arranged to control the switch means when power is removed by: switching the second switch means to switch the inner feedback loop to the voltage-gain amplifier and subsequently opening the first switch means to disconnect the power supply lines from the current-gain amplifier.
As a result of switching the outer feedback loop to the voltage-gain amplifier, the current-gain amplifier is in open loop when the power supply lines are disconnected. As a result, the removal of power from the current-gain amplifier does not create audible noise because its high bandwidth causes any transients to be above the audible range. In contrast, if the current-gain amplifier was in the feedback loop with the voltage-gain amplifier at this time, the low bandwidth of the voltage-gain amplifier would slow the power-down creating audible noise.
Preferably, the audio amplifier circuit further comprises a quiescent voltage supply for supplying a voltage at the quiescent level; and third switch means for selectively connecting the input node of the audio amplifier circuit or the quiescent voltage supply to the input of the voltage-gain amplifier, wherein the control circuit is arranged to control the third switch means when power is removed by: prior to, or on, said switching of the second switch means to switch the inner feedback loop to the voltage-gain amplifier, switching the third switch means to switch the quiescent voltage supply to the input of the voltage-gain amplifier.
This ensures that no audio signals are input to the audio amplifier circuit whilst power is removed, because such audio signals could cause the generation of audible noise as the power level falls.
Preferably, the audio amplifier circuit further comprises a charging path for holding the output node of the audio amplifier circuit to its quiescent level; and a fourth switch means for selectively opening or closing the charging path, wherein the control circuit is arranged to control the fourth switch means when power is removed by: prior to said opening of the first switch means, closing the fourth switch means.
The charging path holds the output node of the audio amplifier circuit at its quiescent level when the power supply lines are disconnected from the current-gain amplifier. This reduces the generation of audible noise when the power supply lines are disconnected.
Desirably, the charging path is connected to a quiescent voltage supply for supplying a voltage at the quiescent level and the control circuit is arranged to control the fourth switch means when power is removed by: subsequent to said opening of the first switch means, opening the fourth switch means to isolate the output node of the audio amplifier circuit from the quiescent voltage supply.
This prevents the fall in the output of quiescent voltage supply from creating audible noise at the output node of the audio amplifier circuit.
Preferably, the audio amplifier circuit further comprises a fifth switch means in series between the voltage-gain amplifier and the current-gain amplifier, and wherein the control means is arranged to control the fifth switch means when power is removed by: subsequent to said opening of the first switch means, opening the fifth switch means to isolate the current-gain amplifier from the voltage-gain amplifier.
This prevents feed-through of a signal from the voltage-gain amplifier during the fall in the level of the output of the power supply. Such a signal might otherwise drive the current-gain amplifier even whilst the power supply is disconnected therefrom and hence create audible noise at the output node.
Preferably, the audio amplifier circuit further comprises a quiescent voltage supply for supplying voltage at the quiescent level to the forward path between the voltage-gain amplifier and the current-gain amplifier; a sixth switch means in series between the quiescent voltage supply and said forward path, wherein the control circuit is arranged to control the sixth switch means when power is removed by: closing the sixth switch means to connect the quiescent voltage supply to the forward path.
According to a second aspect of the present invention, there is provided an audio amplifier circuit comprising: an amplifier circuit between an input node and an output node; power supply lines for supplying power to the audio amplifier circuit; a charging path including a resistance means for charging the output node to its quiescent voltage through the resistance means; charging path switch means for selectively opening and closing the charging path; a power supply switch means for selectively connecting the power supply lines to the output stage of the audio amplifier circuit; and a control circuit for controlling the power supply switch means and the charging path switch means when power is supplied by: initially opening the power supply switch means to disconnect the output stage of the amplifier circuit from the power supply lines; closing the charging path switch means and subsequently, after a delay sufficient for the output node to be charged to its quiescent voltage, opening the charging path switch means and closing the power supply switch means to connect the power supply lines to the output stage of the amplifier circuit.
The charging path allows the output node of the audio amplifier circuit to be charged to its quiescent level. During this charging, the output stage of the audio amplifier circuit is effectively off because its power supply is disconnected. When the power supply is connected to the output stage of the audio amplifier circuit, the amount of noise generated is reduced, because the output node has already been charged to its quiescent level.
The power supply switch means may selectively connect the power supply lines to all of the audio amplifier circuit including elements before the output stage, but preferably this is not the case, so that the remainder of the audio amplifier circuit receives power as soon as power is supplied to the power supply lines. This allows the remainder of the audio amplifier circuit to stabilise to its quiescent operating condition before power is supplied to the output stage.
Desirably, with both the aspects of the present invention, the current-gain amplifier has no inherent DC offset between its input and output. This means that when the power supply is connected to the current-gain amplifier, there is no step in the output voltage which could create an audible noise.
The two aspects of the present invention may be implemented together, as in the hereinafter described embodiment. The various features of the two aspects of the present invention may be freely combined together.
As described above, both aspects of the present invention reduce the audible noise generated during power supply switching.
Both the aspects of the present invention the audio amplifier circuit of the present invention may be implemented in a single integrated circuit chip. This makes it possible to provide an IC chip incorporating an audio amplifier circuit in which audible noise on power supply switching is reduced without the necessity to implement a further circuit external to the IC chip. This avoids the need for further noise reduction techniques to be employed when the IC chip is incorporated into a larger circuit and also minimises the circuit scale.
The audio amplifier circuit may be used in any device where it is necessary to amplify an audio signal. For example, it may be used in an audio-visual switch.
An audio amplifier circuit which embodies both the aspects of the present invention will now be described by way of non-limitative example with reference to the accompanying drawings in which: